AAV is a virus having a linear single-stranded DNA genome of 4.7 kb, comprising open reading frames of two genes rep and cap. The rep gene encodes four proteins necessary for genome replication (Rep78, Rep68, Rep52, and Rep40). The cap gene expresses three capsid proteins that assemble for formation of a viral capsid (VP1, VP2, VP3), and assembly-activating protein (AAP). Replication of AAV in nature relies on the presence of a helper virus such as an adenovirus or a herpes virus. In the absence of a helper virus, the genome of AAV is maintained in an episome or integrated into a chromosome of a host, so that the AAV is present in a latent state. Over one hundred serotypes and clades (non-patent literature 1) of AAV are currently identified. Particularly, development of vectors for gene delivery based on AAV2 is advanced.
In 1989, a gene delivery vector system based on AAV2 was developed for the first time. Vectors based on AAV have been found to have many advantages. Since wild-type AAV is nonpathogenic and has no etiological relation to any known diseases, vectors based on AAV are believed to be extremely safe. In addition, AAV has high gene transduction efficiency.
Administration of AAV particles enables long-period and stable gene transduction into various target organs and target cells. Until now, gene transduction with high efficiency into skeletal muscles, liver (hepatic cells), heart (cardiac muscle cells), nerve cells, pancreatic gland cells, and pancreatic islet cells has been reported. In addition, AAV has been used in human clinical trials. On the other hand, an attempt to change the cell tropism of AAV by alteration of capsid proteins of the AAV and an attempt to avoid removal of AAV particles by neutralizing antibodies have been made. For example, AAV capsids with tropism for specific organs and cells such as neuroglia cells, airway epithelial cells, coronary artery vascular endothelial cells, and lung, and AAV capsids with tropism for tumor cells such as glioblastoma cells, melanoma cells, lung cancer cells, and breast cancer cells have been created (non-patent literature 2).